Identifying Trends for Understanding the Role of Humic the Environmental Behavior of Adionuclides
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[1-12]. The effects of different experimental methods, pH, humic acids of different origin, ionic strength and formation of mixed complexes [13-17] have been studied. The results from a number of these studies have been used to develop a complexation model which gives invariant stability constants [18]. However, humic acid in the environment is not confined to the aqueous phase, often absorbing to inorganic surfaces and colloids [19-22]. Investigations with actinide and lanthanide ions [23-27], and transition metal ions [28-30] verify the humic coating of these surfaces is an important component in metal ion sorption. From these studies pH and ionic strength are important terms in describing the sorption of metal ions to humic coated surfaces, similar to results from aqueous complexation studies. The sorption has been empirically modeled [20,31]. However, with a mechanistic approach experimental results can be universally applied. Consequently an improved understand of the sorption process and a larger data base for modeling can be achieved In this paper, a charge neutralization model is used to analyze humate complexation data. Through this systematic analysis with a consistent model, one is able to identity trend and apply results from different conditions. Experimental results from Gorleben humic acid studies are systematically examined for analyzing the impact of fulvic acid on Am and Cm transport at Oak Ridge National Laboratory. Additionally, studies on humic acid covalently bonded to silica support shows similarities between the complexation behavior of sorbed and aquatic humic acid. 985 Mat. Res. Soc. Symp. Proc. Vol. 556 ©1999 Materials Research Society
COMPLEXATION MODEL Humic substance modeling used in this work is based on the concept of metal ion charge neutralization upon complexation to humic acid functional groups [18]. The complexation constants are derived by introducing the operational humic acid concentration and loading capacity (LC). The loading capacity, which changes with pH, ionic strength, origin of humic acid and metal ion, is the mole fraction of the maximum available complexing sites of humic acid. With the metal ion charge neutralization model complexation constants are invariant regarding experimental conditions. For metal ion charge neutralization the metal ion occupies the number of proton exchange sites equal to its charge. The proton exchange capacity is found by titrating a known amount of humic acid. A group of complexing sites needed to neutralize a metal ion is considered as a one humic acid complexation unit. The reaction is: Mz+ + HA(z) MHA(z)
(1)
The complexation constant is: [MHA(z)]
-
[MZ+
]f [HA(z)]f
(2)
where [MHA(z)] is the concentration of metal ion humate complex, [MZ+]f the free metal ion concentration, and [HA(z)]f the free humic acid concentration. All concentrations under consideration are defined in mol/L. For the metal ion charge neutralization model, [HA(z)]f is calculated using a loading capacity determined by experiment. The value for [HA(z)]f is di
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